Checked array access is implemented with unsafe code. Actual code implementing the `[]` operator for arrays ("slices"): fn index(&self, &index: &uint) ->

Checked array access is implemented with unsafe code. Actual code implementing the `[]` operator for arrays ("slices"): fn index(&self, &index: &uint) -> &T { assert!(index < self.len()); unsafe { mem::transmute(self.repr().data.offset(index as int)) } } By having `unsafe {}` it's possible to build low-level functionality as libraries rather than in the compiler. There no reason to think that code such as this would be less prone to bugs if it were hard-coded into the compiler. I'd be more inclined to believe the opposite.